The present invention relates generally to introduction of a sample to a mass spectrometer (MS) and is more particularly concerned with simplified method and apparatus by which a liquid sample is nebulized into fine particles in extremely efficient manner to facilitate vaporization thereof and the nebulized liquid sample is then continuously introduced to the MS thereby enabling the MS to handle a wider range of compounds for mass spectrometric measurement.
A mass spectrometer (MS) has been widely used as an instrument for performing both qualitative and quantitative analyses of different gases, volatile liquids and solid molecules by obtaining mass spectra thereof through measuring an ion current caused by ions collected on one or a plurality of fixed ion collector electrodes while adjusting or varying the accelerating potential and intensity of the magnetic field. In recent years, it has been attempted to employ such an MS in connection with another type of separating and analyzing instruments for efficient analysis of mixtures.
For example, a liquid chromatograph (LC), especially a high-performance liquid chromatograph (HLC), has attracted a good deal of attention in this field and is presently accepted as an excellent means for separating a mixture into its constituents and determining the individual constituents quantitatively or volumetrically mainly because it is capable of separating either fat- or water-soluble substances into its components, without any preliminary modification of such substances, by simply making a proper choice of a solvent and a separation column. Although such an HLC is given a surpassing capability of separating mixtures per minute, it has a disadvantage that it is not completely satisfactory in its ability of identifying the separated constituents. The mass spectrometer (MS), on the other hand, is extremely high in its sensitivity and performance in identification of a single component but it has also a disadvantage, at the same time, that it is accompanied with considerable difficulty when identifying a mixture because the spectra to be obtained are complicated. To make the effective use of the advantages and make up the disadvantages of both LC and MS instruments, there has been presented a LC-MS analyzing system which is a combination of the two.
Further, the analysis by mass spectrometry with use of the MS encounters various sorts of problems when the sample to be handled is a liquid or a solution of any solvent because the mass spectrometric analysis is achieved by ionizing the sample after it is converted into a gas state. In other words, any sample which is a liquid or solution requires vaporization thereof before it is introduced to an ionizing portion (ion source) of the MS. However, it is an extremely hard practice to attain continuous yet stable vaporization of such a liquid sample and the subsequent introduction thereof to the ion source especially when the sample is a polar or large molecular-weight compound. The compound LC-MS analyzing system previously described also has a potential of suffering from problems similar to those of the MS above indicated because the sample to be introduced to the MS of the system is an effluent from the separation column of the LC, which is a liquid. The most contributing causes preventing practical use of the LC-MS analyzing method are its difficulty in continuously removing a solvent (mobile phase) from a continuous flow of the effluent supplied, and the resultant failure in offering an effective means for concentrating the desired constituent present in the effluent.
To solve such problems as stated above, a variety of methods and procedures for introducing the liquid samples to the MS have been examined and proposed up to the present. Some of the most positively attempted solutions to the problems have been concerned with development of a so-called "interface", i.e. a means for connecting the LC to the MS to establish a LC-MS combined analyzing system, on which the spotlight of research workers' attention has been focused. Up to date, however, only a few reports of research on the LC-MS connecting methods have been made public, including the following:
(1) LC-EIMS Method
The LC-EIMS Method [J. Chromatogr., 99, 395 (1973)], which was developed by R. P. G. Scott et al. is characterized by a fine stainless steel wire traveling through an ion source of the MS wherein the ionization is carried out by an electron impact (EI). Approximately one percent (1%) of the effluent from the MS adheres to a portion of the traveling wire outside the housing of the MS and the solvent in the effluent adhering to the wire is removed during its travel through a preliminary heating and evacuation system whereby only the desired constituent of the effluent is finally introduced into the ion source. To ensure higher sealing performance of individual interface chambers which are in communication with the preliminary heating and evacuation system, the wire is passed through small jewel apertures provided in each of partition walls separating from one chamber to another. Scott et al. state in their report that this method is applied to handling mixtures of several different drugs and metabolites and of fermented products, and that the desired constituent detected and identified by the method ranges from 10.sup.-6 to 10.sup.-7 g per ml of the mixture to be handled. However, the method retain not a few points that should be improved before it is put into practice, including its possibility that the sample being fed by the wire may stick to the jewel apertures or the sample sticking to the apertures may be mixed with the newly fed sample on the wire while the wire is traveling through the apertures, as well as its insufficient capability of quantitative analysis.
(2) Direct Chemical Ionization (DCI) Method
The direct chemical ionization method [Org. Mass Spectrom., 7, 1353(1973)] was developed by McLafferty et al. It is well known that the chemical ionization (CI) method is effective in a combined system of a GC (gas chromatograph) and an MS. In a system of an LC coupled with the MS, however, there is a considerable difficulty in removing entirely the mobile phase of the LC which is a liquid. To overcome this difficulty of the LC-MS system, is proposed this DCI method which is intended to introduce a desired constituent of the effluent from the LC column together with the solvent thereof without previous removal of the latter, and to utilize the solvent vapor as a reagent gas thereby measuring the CI spectra. In the DCI method, an elution speed of the LC and a way of introducing the effluent to the ion source of the MS hold important keys to successful measurement of the CI spectra. But as far as these two conditions are properly established, the CIMS (Chemical Ionization Mass Spectrometer) is more easily connected to the LC than the EIMS (Electron Impact Ionization Mass Spectrometer), and has a potential of continuous measurement of the CI spectra. McLafferty et al. designed the LC-CIMS interface so that approximately one percent (1%) of the effluent from the LC is conducted through a glass capillary tube (0.076 mm in diameter) into the ion source. They applied the interface to a mixture of different steroids each being of 0.8 .mu.mol and obtained a good result. However, this interface method has some disadvantages. At first, amount of effluent that can be used is as less as 1% and in addition, it is very difficult to manufacture the glass capillary which is so small in diameter. Even if it was possible, it would be a hard practice to feed the effluent through such a fine capillary. Another disadvantage of this method is caused by the fact that the effluent is introduced directly into a heated ion source. As a result, the introduction of the effluent is easily and frequently interrupted due to bumping or other phenomena at the ion source. This disadvantage has made it difficult to accomplish a continuous and stable vaporization of the effluent.
As indicated above, any one of the conventionally-available interfaces between the LC and MS instruments has a number of inherent problems such as complicated construction, less ease of operation and extremely high manufacturing cost, and in general, fails to completely satisfy the requirements as a practicable device. The conventional failure in providing the practicable interface device means that there is not yet developed any method and device commonly available for introducing liquid samples to the MS. To put the LC-MS analyzing procedure into practice, it is an urgent matter and need to provide a proper method for the liquid sample introduction to the MS and to develop an apparatus which is simple in construction, easy in operation, high in sample concentrating capability and performance, and low in manufacturing cost.